The present invention relates to the field of mobile radio communications systems, and, more particularly, to the problem of increasing the quality of a wireless transmission without substantially decreasing the overall throughput of the communications channel.
Mobile radio channels are often characterized by the unpredictability of the channel due to, inter alia, Rayleigh fading and long term shadow fading. The channel quality may degrade as a result of several factors, such as, for example, co-channel interference adjacent channel interference propagation path loss, and multi-path propagation (i.e., Rayleigh fading). Transmission errors typically occur in bursts when fading causes the signal level to go below the noise or interference level. Therefore, explicit measures often need to be taken to maintain an acceptable level of quality of the transmission over a radio channel.
The quality of the transmission over a radio channel connection may be measured by the reliability with which the receiver receives the transmitted data. This channel reliability may, for example, be defined in terms of the bit-error-rate (BER) as experienced at the receiver.
Specifically, forward error correction (FEC) and automatic repeat request (ARQ) are two well-known error control techniques commonly used for noisy and fading channels. In a system that uses FEC for error control, for example, the transmitter encodes the data using a given redundancy code, while the receiver, which has been informed of the code used, decodes the data at the receiving end. Many such systems using block or convolutional codes have been explored and/or employed. In a system that uses ARQ, for example, the receiver returns (i.e., transmits back to the transmitter) an acknowledgment which indicates whether the given transmitted packet was received free of errors (in which case an acknowledgment signal, or xe2x80x9cACKxe2x80x9d is sent), or whether it was received erroneously (in which case a negative acknowledgment signal, or xe2x80x9cNACKxe2x80x9d is sent) . If the packet was not received error-free (i.e., if the transmitter receives back a xe2x80x9cNACKxe2x80x9d signal), the transmitter then re-transmits the same packet again, anticipating that the packet will be successfully received on this (or else on a further, subsequent) transmission.
Transmission of multimedia applications such as high quality audio, images and real-time video, for example, require very low bit-error-ratesxe2x80x94typically 10xe2x88x926 or less. obtaining such low BERs in wireless environments is challenging, even in the presence of very low rate forward error correction codes. ARQ techniques, however, provide very reliable communication, albeit at the expense of variable and sometimes large delays. However, hybrid ARQ schemes, in which both FEC and ARQ techniques are employed simultaneously, are particularly attractive because they combine the fixed delay error correction capability of FEC techniques with the low BER of basic ARQ schemes.
The performance gains obtained from an FEC technique depends on the state of the channel. For example, when the received signal-to-noise (SNR) is large, an uncoded system or a high code rate FEC is sufficient to give a satisfactory BER. On the other hand, for lower received SNRs, a very low rate FEC may be necessary to meet the requirements. Adaptive hybrid ARQ schemes can be used in channels with varying BERs, wherein a high rate FEC is used during times when lower BERs are experienced and a low rate FEC is used during times when higher BERs are experienced. In comparison to non-adaptive hybrid ARQ schemes, adaptive schemes employ fewer bits for error correction. Therefore, these adaptive schemes typically result in a better overall throughput than do non-adaptive schemes.
In an adaptive hybrid ARQ, a retransmitted packet may need more redundancy (i.e., a lower coding rate) than the first time sent packet when the channel condition is deteriorating in the retransmission. Since each packet can only carry a fixed number of bits, the information in the corrupted packet may need to be transmitted in more than one packet when a lower coding rate with more overhead is needed in the retransmission.
Traditionally, packets are numbered consecutively based on their transmitted orders and the sequence number for the retransmitted packet is the same as that for the original packet. Such a packet numbering scheme will create problems for an adaptive hybrid-ARQ with selective retransmission, since the original packet may be divided into multiple packets for the retransmission. Where the original packet is divided into multiple packets, there will be a shortage of sequence numbers for the extra packets generated due to the change of coding rate. In an adaptive hybrid-ARQ scheme known as the xe2x80x9cgo-back-Nxe2x80x9d protocol, the corrupted packet, as well as each packet after it, are retransmitted. The foregoing approach, however, reduces channel throughout since properly received packets are unnecessarily retransmitted.
Accordingly, it would be advantageous if the availability of sequence numbers could be assured while selectively retransmitting only corrupted packets in an adaptive hybrid-ARQ coding scheme.
The present invention is directed to a system, method, and apparatus for assuring the availability of sequence numbers in an adaptive hybrid automatic repeat request coding system capable of multiple forward error correction (FEC) rates. Each batch of data received from a data source is assembled into a packet with a header containing a packet number. The packet numbers are incremented by at least the ratio of the amount of data which can be transmitted in the packet using the highest FEC rate and the amount of data which can be transmitted in the packet using the lowest FEC rate.